This invention relates to a valve timing control means for engines and more particularly to an improved and simplified arrangement for changing the valve timing of an internal combustion engine.
As is well known, the valve timing of an internal combustion engine comprises a compromise between the timing which will produce the maximum output and the timing which produces the best idle and low speed running characteristics. Like many other factors in engine design, the valve timing is a compromise between these conditions. If maximum power output is to be attained, there should be a substantial degree of overlap between the opening of the intake valve and the closing the exhaust valve so as to insure good charging of the combustion chamber as well as effectie scavenging at the end of combustion. However, such large overlaps can provide extremely poor running under low speed conditions when the exhaust gases escape back into the intake passage during the overlap condition. Thus, as has been noted, the actual valve timing chosen is a compromise between these two conditions.
It has been proposed to provide some arrangement for altering the valve timing of the engine during its running so that optimum valve timing will be provided at all running conditions. Although in theory this sounds like a good solution to the problem, the mechanisms heretofore proposed for this purpose have been complicated, have added to the cost of the engine and have reduced its durability. For example, one type of valve timing adjusting mechanism adjusts the position of the camshaft relative to its driving sprocket so as to afford timing adjustment. Such arrangements can reduce the loads which can be transmitted through the driving connection and may offer the possibility of premature failure. In addition, these mechanisms are relatively complicated. With another form of device, the flexible transmitter (either a timing chain or cogged timing belt) is subjected to a load by a tensioner-type mechanism so as to adjust the valve timing. These type of devices are simplier than those which operate on the connection between the camshaft and its driving sprocket but they tend to increase or vary the tensioning on the driving chain or belt and can cause premature wear of these components. In addition, this type of device causes the actual tensioning on the chain or belt to vary during running and the jumping of teeth is a distinct possibility which can result in destructive loss of valve timing.
It is, therefore, a principal object of this invention to provide an improved and simplified arrangement for adjusting the valve timing of an engine during its running.
It is a further object of this invention to provide a valve timing adjustment mechanism that will not decrease the durability of the engine.
It is a further object of this invention to provide an adjustable valve timing mechanism that operates on the drive for the camshaft and which does not unduly stress that drive.